Wavelength division multiplexing has become an important tool for increasing the data transmission capacity of fiber optic communication systems and local area networks. A variety of different designs have been proposed for wavelength multiplexer/demultiplexers. These are of particular interest for single-mode fiber telemetry systems because of their higher bandwidth and lower loss.
U.S. Pat. No. 4,834,481 describes one type of single-mode fused coupler which acts as a wavelength multiplexer/demultiplexer at wavelengths of 1.32 .mu.m and 1.55 .mu.m. A tapered fused coupler can be fabricated by bringing two fibers together and then tapering and fusing with an appropriate heat source. This procedure is carried out with a light source coupled into one of the fibers while monitoring the light intensities from output ends of the fibers to determine the amount of coupling. The power transfer between the coupler output ends undergoes sinusoidal oscillations or beats as the tapering process continues and is said to have been pulled through one beat length when the coupled power has cycled through one complete sinusoidal oscillation back to zero. The coupling ratio will be equal to zero when the coupler is pulled through integer multiples of one beat length and will be equal to 100% at half-integer multiples of one beat length. In this particular coupler, the coupler is pulled through 3/2 beat lengths at the 1.32 .mu.m monitoring wavelength in order to obtain 100% and 0% coupling ratios at the respective wavelengths 1.32 .mu.m and 1.55 .mu.m.
Drawbacks of the present methods of making two-channel fused couplers are their empirical nature. For a given type of monomode fiber, the fabrication conditions for the process are adjusted until the correct wavelength response is found. The fabrication of a fused coupler using monomode fibers from a different supplier requires the determination of a new set of fabrication conditions.
A further limitation of present designs for two-channel fused couplers is the isolation between the channels for these couplers. Successful operation of a wavelength division multiplex (WDM) link requires that the cross-talk between the channels is low. The isolation in a fused coupler can be large. However, due to lack of control in the manufacturing process, the 0% and 100% coupling points may not occur at exactly the desired wavelengths and the isolation between the channels can deteriorate considerably. Even if the coupler is made perfectly, crosstalk can result because of variations in the operating wavelengths of commercially available laser light sources. Furthermore, changes in environmental conditions can also shift the operating wavelength of a laser causing the isolation between the channels to deteriorate.